Page 101 - Applied Petroleum Geomechanics

Rock strengths and rock failure criteria 93

Figure 3.6 MohreCoulomb strength envelopes obtained from triaxial compression
tests for weak sandstone at depth of 28,014 ft TVD KB in the Gulf of Mexico (Zhang
et al., 2008).

When a series of triaxial compression tests are conducted, the cohesion
and angle of internal friction of the rock can be obtained from the Mohr
circles and strength envelopes. Fig. 3.6 plots the Mohr circles and the
MohreCoulomb strength envelopes from three triaxial compression tests
and one uniaxial test in a weak sandstone in the Gulf of Mexico. It indicates
that the rock may have different strength envelopes for different confining
stresses. From these tests, the cohesion and angle of internal friction of the
rock can be obtained from the Mohr circles and strength envelopes (refer to
Section 3.4.2). Fig. 3.6 shows that the cohesion (c) is 440 psi, when the
confining stress is less than 6890 psi; whereas, when confining stress is
greater than 6890 psi, the cohesion is significantly higher (c ¼ 5365 psi).
This observation suggests that when confining stress gets higher, the
apparent rock cohesion starts to increase while the angle of internal friction
tends to reduce. For example, if the mud weight is 10 ppg at a depth of
12,000 ft, the mud pressure (confining stress for the wellbore) is about
6234 psi. However, at the depth of 8000 ft, the same mud weight of 10 ppg
is equivalent to a mud pressure of 4156 psi (i.e., a smaller confining stress for
the wellbore). That is, for the same mud weight at a shallower depth, the
wellbore has a lower cohesive strength and may be more likely to expe-
rience shear failures. Certainly, the confining stress is only one of the factors
to impact wellbore stability.